A semiconductor device having a diode and an IGBT is disclosed in, for example, US Patent Application Publication No. 2007/0108468.
In the device, an N type layer and a P type base layer are formed on an N-type substrate. A pair of grooves is formed on a surface of the base layer such that the grooves reach the substrate. An N type emitter region is formed in the base layer such that the emitter region is sandwiched between the grooves.
A gate oxide film is formed on an inner wall of the groove, and a gate electrode is formed in the groove through the gate oxide film. Thus, the base layer contacts the gate electrode via the gate oxide film. The base layer provides a channel region of the IGBT. An interlayer insulation film is formed over the gate electrode such that the interlayer insulation film covers a part of the emitter region.
An emitter electrode is formed on a part of the emitter region and the base layer. The emitter electrode also provides an anode electrode of the diode. A P+ type collector layer and an N+ type cathode layer are formed independently on a back side of the substrate. A collector electrode coupled with both of the collector layer and the cathode layer is formed on the substrate. The collector electrode also provides a cathode electrode of the diode.
The diode and the IGBT are integrated in the substrate. The IGBT region having the collector disposed on the back side of the substrate functions as the IGBT. The diode region having the cathode layer disposed on the back side of the substrate functions as the diode. Multiple diodes and IGBTs may be formed in the substrate so that an inverter is provided.
However, the inventors realize the following difficulty.
When a forward bias of the diode is applied between the emitter electrode and the collector electrode, electrons is supplied from the cathode layer to the substrate. Further, holes are supplied from the base layer to the substrate. Then, the electrons in the substrate are dominant, i.e., excess, so that the electrons flow from the cathode (i.e., the collector electrode) to the anode (i.e., the emitter electrode). Thus, the diode flows the forward current therethrough.
In this case, when a reverse voltage is rapidly applied between the emitter electrode and the collector electrode, the reverse current flows for a short moment. Specifically, the holes supplied from the base layer to the substrate moves toward a direction opposite to the forward direction so that the holes moves to the emitter electrode side. Further, the holes remained in the substrate recombine with the electrons, and/or the holes in the substrate are diffused, so that the reverse current flows, i.e., reverse recovery occurs. Accordingly, the holes accumulated in the substrate flows into the anode (i.e., the emitter electrode) via the base layer at the recovery process.
When the IGBT turns on, since the resistance of the channel region in the IGBT is very small, the emitter electrode and the substrate short-circuit. That is, at the diode recovery operation, when the IGBT turns on, the diode short-circuits, and the current flows from the diode region to the IGBT region.
Accordingly, in a conventional semiconductor device, at the diode recovery operation, the current easily concentrates at the boundary between the diode region and the IGBT region, so that breakdown of the device may occur.
Thus, it is required for a semiconductor device having a diode and an IGBT to prevent current concentration at a boundary between the IGBT and the diode in case of diode recovery process.